1. Field of the Invention
The present invention relates to a disk drive adapted to operate in conjunction with a disk storage medium having a multilayer signal plane, and more particularly to an apparatus and method for controlling the focus of a light beam output from an optical head via its objective lens.
2. Description of the Related Art
In the art of disk storage media, in particular those which have been developed in recent years such as a DVD (digital versatile disk)-ROM, two-layer disks having two signal layers are known. FIG. 1 is a cross-sectional view illustrating an example of the two-layer disk structure.
As shown in FIG. 1, this two-layer disk 100 includes a disk substrate (transparent layer) 101 disposed on the surface side 108 of the disk and made of a transparent synthetic resin having a high light transmittance, high resistance to mechanical shocks, and high chemical resistance such as polycarbonate resin, polyvinyl chloride resin, or acrylic resin.
On the upper surface (on the upper side in FIG. 1) of the disk substrate (transparent layer) 101, there are provided a first signal plane 102 and a first reflection layer 103 corresponding to the first signal plane 102 wherein a first-layer data storage plane (signal layer) is formed with the first signal plane 102 and the first reflection layer 103. Furthermore, there are provided a second signal plane 103 and a second reflection layer 105 corresponding to the second signal plane 104, and a second-layer data storage plane (signal layer) is formed with the second signal plane 103 and the second reflection layer 105. On the second reelection layer 105, there is provided a bonding layer 106. A dummy plate 107 is bonded to the second reflection layer 105 via the connecting layer 106.
The first reflection layer 103 is made of a semitransparent film so that a fixed fraction of a laser beam is reflected by the first reflection layer 103. In the above-described structure, when the laser beam is focused on the first signal plane 102, the signal recorded on the first signal plane 102 is read from light reflected from the first reflection layer 103. When the laser beam is focused on the second signal plane 104, the laser beam passes through the first reflection layer 103 and is then focused on the second signal plane 104. In this case, the signal recorded on the second signal plane 104 is read from light reflected from the second reflection layer 105.
When a recording/reproducing operation is performed using a two-layer disk having the structure described above, it is required to switch the focal position, where the light emitted from an optical head via an objective lens is focused, from the first layer to the second layer or from the second layer to the first layer by switching the conditions of the focusing servo control so that the signal recorded/reproduced operation associated with the first layer is switched to that associated with the second layer or vice versa. The switching of the focal position is herein referred to as focus jumping. The control of the fucus jumping is described below with reference to FIG. 2.
FIG. 2 schematically illustrates the positional relationship between the two-layer disk 100 and the objective lens 2. In FIG. 2, there are shown two possible locations of the objective lens 2 on the surface side of the two-layer disk 100, wherein one location is represented by a solid line and the other is represented by a broken line. When the objective lens 2 is positioned at the location represented by the solid lines relative to the location of the two-layer disk 100, the light beam emerging from the objective lens 2 is focused within a range (first focus-drawing-in range FL1) which allows the objective lens to be drawn to a just focusing point associated with the first layer. If the focusing servo loop is closed in this situation, the location of the objective lens is converged to a location where the light beam is precisely focused on the first signal plane 102.
On the other hand, when the objective lens 2 is at the location represented by the broken line, the light beams is focused within a range (second focus-drawing-in range FL2) which allows the objective lens to be drawn to a just focusing point associated with the second layer. If the focusing servo loop is closed in this situation, the location of the objective lens is converged to a location where the light beam is precisely focused on the second signal plane 104.
When the focal point of the objective lens 2 is switched (jumped up) from the first layer to the second layer, the focus jumping operation is perform in such a manner that the focusing servo loop is opened when the focal point of the objective lens 2 is within the first focus-drawing-in range FL1, and a kick signal (acceleration signal) having a predetermined level is applied to a focus driver so that the objective lens 2 is moved upward (in FIG. 2) by a force applied to the objective lens 2 by the focus driver. In the above operation, the level of the kick signal is set to a value which allow the focal point of the objective lens 2 to move through the first layer into a second focus-drawing-in range FL2 of the second layer without exceeding the second focus-drawing-in range FL2.
At a proper time after that, the kick signal is turned off and a brake signal (deceleration signal) having a proper level is applied to the focus driver so that the objective lens 2 which was driven by the kick signal and which is now moving upward by means of inertia is decelerated.
As a result of the above operation, the objective lens 2 comes to a position within second focus-drawing-in range FL2 and the velocity of the objective lens 2 is reduced to a sufficiently low level by the deceleration given by the brake signal. At a proper time after that, the focusing servo loop is closed (to perform a focus drawing-in operation) thereby starting a focusing servo control operation by which the location of objective lens is converged to a location where the the light beam is focused on the second signal plane 104 of the second layer. In this way, the focal position of the objective lens 2 is switched from the first layer to the second layer.
By performing a similar focus jumping operation in an opposite direction, it is also possible to switch (jump down) the focal position of the objective lens 2 from the second layer to the first layer. In the jumping-down operation, the kick signal and the brake signal have a polarity opposite to that employed in the jumping-up operation.
A specific example of the focus jumping control is disclosed in U.S.patent application Ser. No. 691,136 filed by the present applicant (Jul. 24, 1996).
In practical applications, users do not always place their DVD disk drive or computer including a DVD disk drive in a horizontal position.
FIG. 3 illustrates some examples of positions in which a computer including a disk drive is placed. In the case of FIG. 3A, a computer 200 having a disk drive O disposed in the main part 201 of the computer 200 is placed in a horizontal position. In the case where the computer 200 is placed in such a manner, when a disk D is loaded into the disk drive O, the surface 108 (refer to FIG. 1) of the disk faces the bottom of the main part 201. The objective lens 2 of the optical head is located on the surface side of the disk and is moved in a direction along the height of the main part 201, as represented by an arrow A in FIG. 3, during the operation of controlling the focal point of the objective lens 2. That is, the objective lens 2 is moved in the vertical direction during the focusing control operation when the computer is placed as shown in FIG. 3A. In this case, the direction of the tracking control of the optical head, that is, the direction of the sled motion, is along the depth (denoted by an arrow B in FIG. 3) of the main part 201.
In the example shown in FIG. 3B, the computer is placed in a vertical position such that the left side of the main part 201 faces down. In the case of FIG. 3C, the computer is also placed in a vertical position but such that the right side of the main part 201 faces down. In the positions shown in FIGS. 3B and 3C, the surface of the disk D loaded in the disk drive O is along the vertical direction, and the focusing control of the objective lens 2 is performed in a horizontal direction as represented by arrows A.
In practical situations, users may place their computer in any of the positions shown in FIGS. 3A, 3B, and 3C. Therefore, it is desirable that the disk drive disposed in the computer be guaranteed to work normally in the recording/reproducing operation when the computer is placed in any of the positions shown in FIGS. 3A-3C.
However, a problem occurs in the focus jumping operation as described below, when the normal operation is guaranteed for any of the above positions of the disk drive.
For example, when the computer is placed in the horizontal position as shown in FIG. 3A, the motion of the objective lens 2 in the focus control direction (denoted by the arrow A) is influenced by gravity. More specifically, when the focal position is jumped up from one layer to the other layer, it is required to move the objective lens 2 against gravity. On the other hand, in the jumping-down operation, it is required to move the objective lens 2 in accordance with gravity.
If control parameters such as the absolute levels of the kick and brake signals, the application timing thereof, and the timing of closing the focusing servo loop are set so that the focus jumping-up operation can be performed in a reliable fashion on the assumption that the computer is placed in the horizontal position as shown in FIG. 3A, then for example the kick signal is set to a relatively high level so that the objective lens 2 can be moved from one layer to the other layer against gravity.
However, if the same control parameters optimized for the focus jumping-up operation are employed in the focus jumping-down operation, there is a high possibility that the focus jumping-down operation fails. That is, the objective lens 2 is kicked, by the kick signal having the relatively high level, in the same direction as the direction of gravity. As a result, the acceleration becomes greater than the optimum value, and the objective lens 2 cannot be decelerated properly in the braking operation following the accelerating operation. Therefore, when the servo loop is closed, the objective lens 2 is probably at a location outside the first focus-drawing-in range FL1 after being moved through the first focus-drawing-in range FL1.
Conversely, if the jumping-up operation is performed using the same control parameters as those optimized for the jumping-down operation, the acceleration of the objective lens 2 becomes smaller than the optimum value and the objective lens 2 does not reach the second focus-drawing-in range FL2. As a result, the objective lens 2 is drawn back into the first focus-drawing-in range FL1, and thus the focus jumping operation fails.
When the computer is placed in the horizontal position, as can be understood from the above discussion, it is required to set the kick and brake signals to different values depending on whether the jumping-up operation or the jumping-down operation is performed, and it is also required to properly set the timing of applying the kick and brake signals and the timing of closing the servo loop depending on the direction of the focus jumping operation.
On the other hand, when the computer is placed in the vertical position as shown in FIG. 3B or 3C, gravitation has no effect on the focus jumping operation regardless of the moving direction of the objective lens 2. However, when the levels of the kick and brake signals, the timing of applying the kick and brake signals, and the timing of closing the servo loop are set on the assumption that the computer is placed in the horizontal position as shown in FIG. 3A, if the computer is placed in the vertical position as shown in FIG. 3B or 3C and the focus jumping operation is performed using the control parameters determined on the assumption that the computer is placed in the horizontal position, there is a high possibility that the objective lens does not reach the desired focus-drawing-in range associated with the desired signal layer and is drawn back into the previous focus-drawing-in range or the objective lens exceeds the desired focus-drawing-in range and thus the focus jumping operation fails.
As can be understood from the above discussion, if the control parameters are set to fixed values, it is difficult to perform the focus jumping operation in a reliable fashion regardless of the position in which the computer is placed.
Even if it is possible to set the control parameters to fixed values so that high-reliable focus jumping operation can be performed regardless of the position in which the computer is placed, there are still other factors which can influence the focus jumping operation. Such factors include a sensitivity variation of a biaxial driver (focus driver and tracking driver) for driving the objective lens 2 and a variation in the distance between two layers of the two-layer disk. Thus it is very difficult to determine the parameters so that the focus jumping operation can be performed in a reliable fashion regardless of any factor.
If the focus jumping operation fails, the operation is repeated until a successful operation is achieved. This causes the user to wait for a long time.